Evaluation of factors affecting bonding performance of silicone structural sealants.

1. The Preface

Glass curtain walls are widely used in modern industry and construction, especially in super high-rise buildings and large commercial buildings. Glass curtain walls not only play a very important decorative role, but also perform well in waterproofing, thermal insulation, and environmental protection. At this stage, glass curtain walls are mostly bonded with silicone structural sealants. In the curtain wall structure, glass, metal frames, plates, etc. are less affected by external factors and have a longer service life, and they play a bonding role. Silicone structural sealant is affected by external factors, including ultraviolet rays, moisture, high temperature, and external forces. Physical or chemical changes occur internally, reducing its original performance and affecting its service life. Therefore, to further improve the service life and structural stability of building curtain walls, it is necessary to conduct a detailed analysis of the factors affecting the bonding performance of silicone structural sealants, and obtain through scientific experiments the effects of silicone structural sealants on ultraviolet radiation, moisture, high temperature, and tension. The attenuation value of bonding performance under conditions such as elongation can be used to understand the aging law of silicone structural sealants and optimize and upgrade them in a targeted manner.

2. Evaluation of quality issues of silicone structural sealants

In building curtain wall structures, silicone structural sealant is a very important adhesive. Using silicone structural sealant, the glass plate and the metal frame can be bonded together, which not only plays the role of sealing, heat preservation, and waterproofing but also can bear the stress of the curtain wall structure. In the building curtain wall structure, the physical properties of the glass plate and metal frame are relatively stable. The molecular structure of the silicone structural adhesive is easily affected by external factors, which reduces its bonding performance and causes aging and cracking, which affects the service life of the building curtain wall and security.

^{Figure 1 The silicone structural sealant of building curtain wall}

2.1 The colloidal aging

The aging of silicone structural sealant is an important factor affecting the service life of building curtain walls, but there are many factors leading to its aging, so its use environment is relatively harsh. Silicone structural sealants are easily affected by environmental factors such as light, rain, snow, and temperature, including high temperature, humidity, ultraviolet rays, mechanical stress, etc., resulting in physical and chemical changes, which in turn leads to a decrease in colloid bonding performance. When surface cracks appear in the silicone sealant, rainwater will enter the curtain wall through the gaps. Long-term accumulation will cause the sealant to hydrolyze, affecting the bonding performance of the silicone structural sealant and reducing its service life. Therefore, the aging problem of silicone structural sealants used in building curtain walls should attract sufficient attention.

2.2 The quality issues

"Silicone Structural Sealant for Buildings" clearly stipulates the quality of silicone structural sealant, which appears as a fine, uniform paste or viscous liquid. The formula of silicone structural sealant directly affects its bonding performance. To pursue profits, some businesses use unconventional means to manufacture silicone sealants and obtain profits by reducing production costs, which seriously affects the quality and bonding performance of silicone structural sealants. Inferior raw materials and inferior additives such as white oil and cracked silicone oil will improve the appearance characteristics of the colloid, but will not help the bonding performance. Although these inferior additives evaporate slowly, once they evaporate, the sealant will crack, pulverize, brittle, and other problems, leading to premature failure of the sealant, causing water leakage, air leakage, increased energy consumption, and other quality problems in the curtain wall. This problem seriously affects the service life of the building curtain wall.

2.3 The colloidal pollution

The formulas of silicone structural sealants vary. To improve the bonding performance of the colloid, some manufacturers will use additives to improve the bonding performance of the sealant and extend the service life of the building curtain wall. The composition or content of the additives will cause pollution to the silicone structural sealant, causing the colloid to embrittle and crack. The mineral components in it will penetrate the building curtain wall panels and cause pollution, especially in super high-rise and large buildings, where the pollution will not occur. It is easy to clean, which not only affects the appearance of the building but also wastes a lot of manpower and material resources because it needs to be replaced.

3. The aging test and results of silicone structural sealant

The aging test of silicone structural sealant mainly studies the effects of ultraviolet radiation, moisture, high temperature, and external force stretching on the tensile bonding properties of silicone structural sealant. Four common silicone structural sealants on the market were selected for testing, and the samples were divided into single-number groups (No. 1, 3, 5, and 7) and double-number groups (No. 2, 4, 6, and 8 No.), prepare samples according to the "Experimental Methods for Building Sealant Materials". Make 25 samples for each number, 5 samples are one group, and the base materials are glass plates and aluminum plates.

An ultraviolet artificial aging test was conducted on silicone structural sealant samples to observe the decay pattern of the tensile bonding properties of the samples. The ultraviolet light test uses a UV fluorescence aging test chamber, the tensile strength test uses an AGIS 100kN universal material testing machine, and the thermal aging test uses a blower with a temperature of 85°C; every 12 hours is a test cycle, and the test times are 300h and 1000h respectively, 2000h, 3000h, 5000h; Stretch the sample to failure at a speed of 5mm/min, measure the length, width, and height of each sample, and test the maximum bonding strength and elongation at the maximum bonding strength. The test results are as follows.

3.1 Photoaging test results of silicone structural sealant

The bonding strength of single-number group silicone structural sealant test samples under ultraviolet radiation was tested separately. When the length reached 1000h, the bonding performance declined significantly, reaching the lowest value at 5000h, with No. 1 and No. 3 showing a linear downward trend. , the downward trend of samples No. 5 and No. 7 is more obvious at 1000h, and the degree of decrease after 3000h is smaller. In the same way, the bonding strength of the silicone structural sealant test samples of the No. 2 group was tested under ultraviolet radiation. It was found that the bonding properties of No. 6 and No. 8 samples decreased at the beginning of the test, and the degradation began to dominate, affecting the silicone structural sealant. Elongation of ketone structural sealants.

3.2 Thermal aging test results of silicone structural sealant

During the test, the silicone sealant samples were taken out of the thermal aging box at 300h, 1000h, 2000h, 3000h, and 5000h to test the bonding strength and elongation. At a temperature of 85°C, the bonding strength of the single-number group test samples showed a functional relationship with the increase of time. Within the test time of 1000h~2000h, the bonding strength of the sample reached the maximum value, but as the time continued to increase to 5000h, the bonding strength of samples No. 1 and 3 began to decrease to the initial value, and the bonding strength of sample No. 5 There was a significant increase, and its value exceeded 15% of the initial value. The bonding strength of sample No. 7 decreased by 10% compared with the initial value. During the thermal aging test of the double-number group, when the time is 1000h, the bonding strength of samples No. 2, 6, and 8 gradually increased, among which samples No. 2 and No. 8 reached the maximum value at 2000h. As the aging test time increases, the bonding strength begins to decrease; Sample No. 4 showed a linear downward trend during the aging test; after 5000h, the bonding performance of the four groups of silicone structural sealant samples dropped to the initial value, among which the bonding performance of Sample No. 4 declined. At most, it is 40% lower than the initial value. 

At a temperature of 85°C, the elongation of the single-number group test samples was tested. When the time reached 1000h, the elongation of samples No. 1, 3, and 5 began to decrease significantly until reaching the lowest value; when reaching 5000h, the elongation rate increased slightly but was lower than the initial value. The elongation rate of sample No. 7 had no obvious change after 5000h. The elongation rate test was conducted on the samples of the double-number group. At the beginning of the test, the elongation rate of samples No. 2 and No. 6 increased slightly, and the elongation rate of the sample gradually decreased at 500h. By 1000h, the elongation rate of the sample decreased steadily, the elongation dropped to the lowest value after 5000h, among which the elongation of sample No. 8 dropped 54% from the initial value, and sample No. 4 showed a linear downward trend in the elongation test.

It can be concluded from the test that high temperature and humidity are important factors leading to the aging of silicone structural sealant. Under the influence of temperature, the molecular structure of silicone structural sealant will undergo violent movements. In the early stage of the aging test, cross-linking reaction is the main factor. The elongation gradually increases. As the aging time continues to increase, the cross-linking reaction causes the molecular chain to continuously lengthen, changing its mechanical properties, thus affecting the elongation. Silicone structural sealant has good heat resistance, and the bonding performance of silicone structural sealant can maintain effective growth at high temperatures. However, because the formulas of silicone structural sealants on the market are different, the test results are certain differences. The standard requirements for the physical properties of silicone structural sealants in "Silicone Structural Sealants for Buildings" are shown in Table 1.

Table 1 The physical performance standards for silicone structural sealants

4. The quality control strategies for silicone structural sealants

Silicone structural sealant is an indispensable adhesive in modern construction and requires very strict quality control during its use. "Silicone Structural Sealant for Construction" clearly states that the colloid elastic recovery rate must reach more than 95%, the tensile bond strength retention rate must be more than 75%, and it must be resistant to high temperatures and will not accelerate aging due to environmental factors, to ensure that the building Air tightness, water tightness and other properties of the curtain wall. The use environment of silicone structural sealant is relatively special. Among its many performance factors, high temperature has the most prominent impact on the bonding performance of silicone structural sealant. In the test, the internal molecular structure of the silicone structural sealant will change at 85°C. Changes occur, and the degree of aging will accelerate with time. When the temperature rises to 95°C, the silicone sealant will fail.

Therefore, the use of silicone structural sealants needs to be carefully selected in high-temperature environments.

With the continuous development of modern industrial technology, the formula of silicone structural sealant has been continuously upgraded. The additive ingredients will directly affect its quality. Therefore, the structure and composition of silicone structural sealant should be strictly controlled in conjunction with the "Silicone Structural Sealant for Construction" By quantitatively or qualitatively detecting the content of alkane plasticizers such as white oil in silicone sealants, we can effectively prevent the breeding of inferior products and extend the service life of silicone sealants.

5. The conclusion

To sum up, the application of building curtain walls in modern urban buildings is becoming more and more common. The curtain wall structure not only has a decorative effect but also plays a very important role in thermal insulation, environmental protection, sealing, etc. Among them, the quality of silicone structural sealant has a great influence on The service life of building curtain walls has the most direct impact. The aging of silicone structural sealants will affect the bonding performance, leading to colloid cracking, curtain wall structure leakage, glass panels falling off, etc., thus causing serious safety accidents. To effectively extend the service life of building curtain walls, it is necessary to conduct a detailed analysis of the aging factors of silicone structural sealants. At this stage, light aging and thermal aging are common aging factors of silicone structural sealants. This article selects four kinds of silicone structural sealants for aging tests. Combined with the "Silicone Structural Sealants for Buildings" standard, the silicone structural sealants are after a detailed analysis of the performance influencing factors, it was concluded that high-temperature conditions have a great impact on the bonding performance of silicone structural sealants. Long-term sunlight will increase the surface temperature of the sealant, making it soft and reducing the bonding performance. When the temperature rises to 95°C, the silicone sealant will fail.

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